1. Field of the Invention
This invention generally relates to improvements in data storage devices for the input and output of information to a data processing system and, more particularly, to a dual copy function in a disk storage device. Each write to the primary storage device is also written to a secondary storage device. Tables are employed to manage the records to be updated and their locations on the physical medium. A status array is also employed to retain the status and device identification information on a status track of each of the data storage devices and another location to provide global identification and management of interchangeable data storage devices.
2. Description of the Prior Art
The past several years have seen the growth of on-line workstations, the evolution of distributed processing, and the acceleration of information technology in many new application areas. The result has been an increase in the use of on-line data base systems and a growth in the requirement for storage capacity and increased reliability and flexibility in data storage devices.
The concepts of self-test, redundancy, cross-check verification between various sources of information and the like are well known in the art, particularly with the advent of complex digital computer systems used for applications such as process control or supervisory control. An example of such a system is illustrated in U.S. Pat. No. 4,032,757 to Eccles, which uses a pair of channels to continually compare the events occurring in each computer. The constant comparison allows the additional processor to quickly take over control of the process if the other processor fails. The problem with this approach is the time that the extra processor needs to begin processing after a failure. In critical processes such as a nuclear power plant, any time lag could be unacceptable. Another approach is presented in U.S. Pat. No. 4,270,168 to Murphy et al., which uses a plurality of processors, self checks and joint answer checking to assure that each processor can assume real time utilization for another processor in the event of a failure. The increased reliability presented in these systems is a memory resident application approach that fails to handle a large data base application spanning many data storage devices.
The general configuration of a data processing system typically comprises a host processor or processors, a memory and various peripheral units. The peripheral units include terminals, printers, communications devices and direct access storage devices (DASD). We are concerned with the control that provides information from DASD to a data base application residing in the host processor memory. Good examples of prior art approaches to this type of processing are presented in U.S. Pat. No. 3,999,163 to Levy et al., U.S. Pat. No. 4,067,059 to Derchak and U.S. Pat. No. 4,189,769 to Cook et al. These patents present various ways to enable a host to process information residing on DASD. While production systems described readily lend themselves to data base applications, they are lacking the capability of retaining status information when a power-off occurs in an environment designed to provide high availability of DASD information.
In a known data processing system, a memory control circuit connected to each of a plurality of memory banks selectively transfers the contents of an arbitrary first one of the memory banks to a selected second one of the memory banks in a manner whereby, if a first memory bank is written into, the utilizing circuit transfers the contents into a second memory bank thereby preventing a loss of information. An example of such a system is illustrated in U.S. Pat. No. 3,866,182 to Yamada et al.